Improvements in and relating to stressed concrete slab structures such as airfield runways and the like



g3 Invenfor Donovan H Lee Oct. 9, 1962 D. H. LEE

IMPROVEMENTS IN AND RELATING TO STRESSED CONCRETE SLAB STRUCTURES SUCH AS AIRFIELD RUNWAYS AND THE LIKE Filed March 20, 1959 ilnited States Patent @fifice 3,057,270 Patented Oct. 9, 1962 3,057,270 IMPROVEMENTS IN AND RELATING TO STRESSED CONCRETE SLAB STRUCTURES ilfigiil AS AIRFIELD RUNWAYS AND THE Donovan Henry Lee, 66 Victoria St., London, SW 1, England Filed Mar. 20, 1959, Ser. No. 800,833 Claims priority, application Great Britain Mar. 24, 1958 4 Claims. (Cl. 94-22) This invention relates to stressed concrete slab structures such as airfield runways and the like of the type wherein the structure comprises one or a plurality of abutting slabs disposed between anchorage means at each end of the structure and stressed by forces applied at the joints between adjacent slabs, the joints being subsequently filled.

In accordance with present practice when stressed concrete is used in such a manner, friction between the underside of the slabs and the material on which it bears, e.g. the ground, is much greater than is desirable for relatively and distributed movement of the slabs over their surface area so that the effectiveness of the stress is considerably reduced. Further, the friction between the slab and the ground prevents the proper distribution of stresses created by shrinkage of the concrete and by thermal effects.

In accordance with this invention the weight of the or each slab is taken by jacking means after the stress has been applied, or while the stress is being applied, or if desired, on subsequent occasions.

According to the present invention, the slab is lifted by fluid pumped under the slab through holes in the slab. To this end a membrane bag may be provided beneath the slab to retain the fluid beneath the slab. Where the fluid is a liquid, e.g. water, the bag is preferably pre-filled with coarse sand or the like. Means are provided in association with the membrane bag to permit the escape of fluid therefrom after lift of the slab and the necessary tensioning has been completed.

In the accompanying drawings:

FIGURE 1 is a sectional side elevation of part of a concrete slab showing a hydraulic or pneumatic means of lifting or jacking up the slab according to the present invention, and

FIGURE 2 is a diagrammatic plan view of a composite concrete slab structure showing the application of stressing forces and the lifting or jacking forces.

As an example of lifting the slabs, according to the present invention, water is injected under pressure underneath each slab 2 through tubes 35a lining holes 35 in the slab, by providing non-return valves 36 (comprising for example spring-loaded ball valve member 36;! and a valve seat 3612) connected to the holes through the slab and connecting a pressure hose 37 to the valve 36. In order to prevent escape of the water laterally of the slab and downward into the ground, a thin waterproof membrane 38, for example, a polyethylene sheet, is laid over the ground prior to commencing concreting and its edges turned back inwardly so that the margins of the waterproof membrane 38 are enclosed and embedded in the concrete around the lower peripheral margins of the slab so as to provide, in effect, a bag.

In practice it is preferred to place on top of the membrane 38 on the ground a layer of fairly coarse sand 39 lacking in fines to a depth of say 1 /2 inches and cover this with a layer of building paper 40 on which the slab 2 itself is cast. The membrane 38 is turned back inwardly at its margins as at 41, as mentioned above so that its edges overlie the margins of the building paper 40 whereby when the concrete is cast the margins of the membrane 38 are trapped between the sand-backed building paper 40 and the concrete. It is also desirable to provide means for draining the water from below the slab after the lifting operation and to this end a number of drainpipes 42 are provided around the periphery or along the sides of the slab to lead to the nearest suitable land drains, the inward ends of the drainpipes 42 being protected by gauze 43 or the like to prevent clogging and the outward ends provided with temporary removable closure means 44. In order to pierce the layer of building paper 40 to permit ingress of air to the sand filled membrane beneath the slab, a pointed mandrel indicated at 45 may be initially passed down each tube 35a lining each hole 35. It is also preferred to lay a thin metal sheet 46 directly below each hole 35 and over the membrane 38 to protect the latter.

In FIGURE 2 there is shown diagrammatically a plurality of concrete slabs A, B and C laid between end anchorages X comprising massive buried blocks or the equivalent. In the junction gap 50 between each slab A and B and B and C, a plurality of stressing jacks 51 or equivalent means are provided which by expansion compress the slabs against the reaction of the end anchorages X to stress the slab-s, all in known manner. The number and spacing of the jacks will depend on the size of the slabs as is well known. According to the in vention as previously described, a membrane bag is located beneath each slab and is filled with fluid under pressure from a source S by means of pipes P connected to the valves 36 each at the head of the tube 35a passing through the slab (see FIG. 1). While FIG. 2 shows four pipes P connected to slab A, the number of such pipes P will depend on the slab area. By this arrangement the slab is lifted from the underlying ground so facilitating the even distribution throughout the slab of the stresses set up in the slab by the compression thereof produced by the stressing jacks 51.

The fluid under pressure in the membrane bag beneath each slab may be released by means of fluid escape drainpipes 42, see also FIG. 1 as already described.

To carry out the stressing operation, the jacks 51 are operated in the junction gaps 50 to compress the adjacent slabs; fluid, e.g. water, is pumped into .the membrane bag(s) beneath the slab(s) so that the or each slab is raised off the ground. When stressing by the jacks 51 has been completed, the stress is held in the slabs in known manner, e.g. by filling in the gaps 50 with concrete between the jacks 51 which can be removed after the concrete has set. The holes left by the jacks can then be filled in. After the jacks 51 have completed their stressing function, or at any suitable time thereafter, the fluid in the membrane bag(s) can be allowed or made to escape so that the or each sl-ab is lowered back on to the ground.

As an alternative compressed air may be used instead of water in the method described in the preceding paragraph and the non-return valves in this case may be motor-car tyre valves or the like.

I claim:

1. In the stressing of concrete slab structures such as airfield runways, roads or the like, of the type indicated, the steps of disposing means forming a membrane bag over the ground area to be covered by the slab, casting the concrete slab in situ, incorporating in the slab one or more tubes passing through the thickness of the slab, pumping fluid under pressure through the tube or tubes into the membrane bag means to cause the fluid to lift the slab, applying a poststress to the ends of the slab after the slab has commenced to be lifted and draining the bag means to deflate the bag and lower the slab after stressing is completed.

2. The method of stressing concrete slab structures according to claim 1, wherein the membrane bag means is filled with a layer of coarse sand or the like prior to casting the slab thereover.

3. The method of stressing concrete slab structures according to claim 1, wherein non-return valve means are secured to each tube at its end adjacent the upper surface of the slab and means are provided on the or each nonreturn valve for connecting it to a supply of fluid under pressure, the arrangement being such that the non-return valve permits fluid to be pumped through to the underside of the slab While preventing the back-flow of fluid from the tube.

4. The method of stressing concrete slab structures according to claim 1, wherein the membrane bag means is provided with openable outlet means located marginally of the slab to permit drainage of the fluid from the bag means after the slab-lifting and stressing operations have been completed.

UNITED STATES PATENTS Albee Feb. 12, Poulter Aug. 7, Poulter Sept. 18, Freyssinet Dec. 24, Freyssinet Oct. 20, Freyssinet July 24, Dobell May 6, Dobell Sept. 23, Freyssinet Nov. 3,

OTHER REFERENCES Product Engineering, volume 29, No. 10, page facing page 60, March 10, 1958. 

